1. Field of the Invention
This invention relates to safety devices for the control of burner fuel supplies and finds particular application in a fail-safe control for a gas burner.
2. Description of the Prior Art
With devices such as water bath heaters employing a burner as the source of heat it is customary to utilise a flame-presence detector to cut off the fuel supply in the event of flame failure. One form of control system used at present relies on thermocouples as flame detectors, but these have a long delay (or drop-out) time (of the order of 30-60 seconds) between flame failure and a positive indication. Current codes of practice demand a maximum total drop-out time of three seconds for self-checking detectors (including the checking time) or one second for non-self-checking detectors and thermocouple controlled safety devices are generally no longer considered satisfactory for industrial applications.
Alternative safety devices which have a faster response time rely on the fact that a burning flame contains ions and is thus electrically conductive. When the flame is burning, a current can be made to flow between electrodes in contact with the flame and detection of this current provides an indication of satisfactory functioning. With asymmetric electrodes, current flow is not the same in both directions and a rectification effect is observed. This can be utilised to provide a more precise safety device, but it is still necessary to provide ancillary circuits to ensure that the operation is fail-safe, that is, any flame failure resulting in cut-off of the fuel supply.